Janus kinases (JAKs) are non-receptor protein tyrosine kinases playing a critical role in cytokine receptor signaling in blood formation, immune responses and in several other physiological responses. The mammalian JAK protein family consists of four members, i.e. JAK1, JAK2, JAK3, and TYK2. The JAK kinases mediate the signaling of all receptors belonging to hematopoietic cytokine receptor type I and type II superfamily and they are required for the biological responses of interferons, most interleukins and colony stimulating factors, as well as hormones such as erythropoietin, thrombopoietin, growth hormone, prolactin and leptin. Due to these fundamental biological properties several cytokines such as erythropoietin, thrombopoietin, growth hormone, granulocyte- and granulocyte-macrophage colony stimulating factors, interferons, and various interleukins are used as drugs to treat human diseases such as anemia, thrombocythemia, immunological diseases, infectious diseases and certain types of cancer. On the other hand, inhibition of JAK kinases is a potent way to treat various diseases where JAK kinases are causing the disease.
The JAK proteins comprise seven different conserved domains (JAK homology domains, JH1-7) (Rane et al., Oncogene 2000, 19, 5662-5679). The carboxyl terminus contains two nearly identical domains, an active kinase domain (JH1) and a catalytically inactive pseudokinase domain (JH2) also termed as kinase-like domain (KLD). It has been generally acknowledged that JH2 lacks enzymatic activity yet it is involved in regulating the activity of JH1. Both biochemical and cell biological data as well as genetic evidence from human diseases and animal models indicate that JH2 has a dual function in regulation of cytokine signaling. JH2 is required to maintain JAK kinases inactive in the absence of cytokine stimulation, but they are also required for cytokine induced signaling. The region immediately N-terminal to the JH2 is a SH2-like domain consisting of the whole JH3 and a part of JH4. The region immediately N-terminal to the SH2-like domain is a FERM-like domain consisting of a part of JH4 and the whole JH5-JH7. Like most kinases, JAKs require autophosphorylation for their full activity. In the case of JAK2, the phosphorylation of the activation loop tyrosines 1007 and 1008 are critical for the activity.
Mutant JAKs are involved in various human pathologies including severe combined immunodeficiency (SCID) and many myeloproliferative neoplasms (MPNs) as well as different leukemias and immunological diseases. Polycythemia vera (PV) is a myeloproliferative disorder that is in most cases caused by a single point mutation in the JH2 domain (JAK2V617F) of JAK2 resulting in aberrant JAK2 signaling, erythrocyte overproduction, and a propensity for thrombosis, progression to myelofibrosis, or leading to leukaemia. The mutation constitutively activates the JAK2 tyrosine kinase and is found in majority of patients with PV and approximately 50% of patients with essential thrombocythemia (ET) and idiopathic myelofibrosis (IMF). Also other less frequent disease causing mutations in the JH2 of JAK2 have been identified in MPN patients. In addition, mutations in the JH2 domains of JAK1, JAK3 and TYK2 are linked with human diseases, particularly hematological and immunological diseases. The dual functional role of JH2 is also demonstrated in these disease associations, and gain of function mutations in JH2 cause hyperactivation of signaling and diseases related to that such as leukemias and cancer, while loss of function abrogate signaling and results in diseases like SCID. The mechanism underlying JAK activation is currently not known.
MPNs are chronic conditions that currently lack specific treatments and the management of these diseases is targeted to the alleviation of symptoms and prevention of complications associated with the conditions. For example, hydroxyurea is commonly used for reducing the number of plateles in patients with PV or ET. However, long term use of hydroxyurea is associated with an increased risk for the development of leukemia. Another commonly used agent for the management of PV and ET is anagrelide which, however, is associated with many side effects.
Increasing effort has been put on the design of MPN specific medicaments many of which are JAK inhibitors. For example, international patent publication WO 2008/057233 discloses a selective JAK2 inhibitor, TG101348, which is currently in clinical trials for the range of diseases caused by V617F-related MPNs. Further, JAK3 inhibitor CP-690,550 is on clinical trials for rheumatoid arthritis, psoriasis, transplant rejection, inflammatory bowel disease and dry eye (R Riese, S. Krishnaswami, J. Kremer (2010) Best Pract. & Res Clin Rheumatol., 24, 513). Other therapeutically potential JAK inhibitors have been disclosed e.g. in US 2009/318405, US 2007/135461, and US 2007/149506. All these inhibitors target the JH1 domain, and they are not able to discriminate between normal and mutated JAK kinase. Given the severity of MPNs and lack of clinically approved specific treatments, there is a great need for the design of further MPN and other cytokine signaling specific medicaments.